Rotaviruses undergo genetic variation by one of two mechanisms: a) accumulation of successive mutations within specific genes (genetic drift); and b) gene reassortment (genetic shift) that results in the appearance of rotavirus strains with a constellation of genes which are derived from two or more distinct rotaviruses. The relative importance of these two mechanisms (genetic drift or shift) in the generation of new strains is not known. Partial sequence analysis of nosocomial rotavirus strains recovered from neonates with asymptomatic infection suggests that the rotavirus genome does not undergo a high rate of spontaneous mutation; on the other hand, rotavirus strains have been identified that appear to have arisen by gene reassortment. In studies in which the RNAs of rotaviruses isolated from asymptomatic newborn infants ("nursery strains") were compared by RNA-RNA hybridization with strains from ill infants a marked difference in the sequence of the fourth gene was detected. Whereas, the fourth gene is highly conserved among the nursery strains, it differs significantly from the corresponding conserved gene of strains isolated from ill infants. These differences in the fourth gene occur independent of serotype (VP7), subgroup specificity (VP6) or the remaining genes that code for structural or non-structural proteins. It is likely that the reduced virulence of the "nursery rotavirus strains" is determined entirely or in part by the unique conserved sequences of the fourth gene of these viruses. This relationship has important implications for the future design of safe, effective rotavirus vaccines.